Help! 280zx turbo is sick

Hello Z people. I have what seems to be a problem I just cant solve. I am working on a 1983 280zx turbo, 5 speed, federal model. It is clean and looks well taken care of, all original. The car is new to me, and has a problem. While driving it will suddenly lose power, stumble, and backfire. It will only do this at light throttle pressure or cruising in 2-3,000 RPM. If I press the throttle harder or completely let off, the problem is gone and the car acts normal. I have read all kinds of posts from a dozen or more websites. I tried all the usual things and I will try to list all I can remember. Vacuum pressure at idle is 20, so no big leaks. Hoses look new and feel supple. Fuel pressure numbers checked out and considered good. Went throuh a tank of Chevron 93 octane gas. Cleaned electrical connections under hood and at ECU. New fuel filter, battery, alternator. Removed clogged catalytic converter. Swapped in a spare TPS. I went for help to an elder at a Z car shop. He sent the ECU to get rebuilt, tuned the car, and made a test pipe which the welds didnt hold so my muffler swings around. The tuner was dumbfounded and was going to swap in an AFM as a guess. He did offer some advice as I was leaving to never come back: Just dont drive in that RPM range. Great huh? So now a large chunk of funds are gone with the wind. So by now I have cleaned connections 3-4 times. Went forward with a tune up. New NGK plugs, cap, rotor, PCV valve. Did the one by one pull on the injector wires and all cylinders are firing. Desperate now I pull the cover off the afm and move the carbon trace board around and try several locations. Nothing. The problem persists. So I fall into depression and just walk away for a bit. Eventually I snap out of it and buy a multimeter. I pull up the 81 turbo supplement FSM and do the AFM check. First at the ECU pins. Range should be 240-400 ohms. I get 199, a fail. Next it says test at the AFM. 199, fails again. It says replace AFM. So excited now, I buy an AFM from the classifieds. I test it. 201 ohms, fail. Tried it anyway. Problem still there. Now... it may seem like I may know what I am doing, but I am in the dark, way out of my element. I need some advice on what to do next. I am lost. Please help.

how did you replace the TPS without a multimeter? you should check it's adjustment. Also recheck the connectors at the ECU. They are problematic and cause weird behavior. Try wiggling them as you drive. I replaced mine with the plugs from a z31. I removed each pin and relocated it to a bare z31 connector. Not too much trouble.
Where's the timing set with the engine at operating temp?

how did you replace the TPS without a multimeter? you should check it's adjustment. Also recheck the connectors at the ECU. They are problematic and cause weird behavior. Try wiggling them as you drive. I replaced mine with the plugs from a z31. I removed each pin and relocated it to a bare z31 connector. Not too much trouble. Where's the timing set with the engine at operating temp?

I had a spare TPS from a 1981, which I just used as a tester, but removed it. Its just a 2 wire on/off style on the turbo. At the time I tested it with key on, and pressing the linkage with a test light. For fun I adjusted it to different settings to look for a change. I may as well do whatever the supplement says to test it. Its been high on my suspect list. ECU connection too. Is that the only way to test them? I can try a wiggle test again, could not hurt. I am interested in how the wire terminals can be released from the connector housing. Im not sure what to use. A needle, cat whisker? I would try the 300zx ECU plugs at this point. The timing...the tuner said it was fine. I dont know much about it on the turbo. I dont have a timing light. Something to read about for sure. I forgot to mention my skill level is middle low under the hood. What do you think?

Went ahead with the TPS. It passed the 2 tests at the ECU, and the part itself. It then said to go to the TVS section and follow directions for adjustment. I adjusted it with the multimeter method. Turning it untill it was in the off position where there is no reading. Let the car get up to temperature and the drive was the same. Accelerates/deccelerates fine but still stumbles and backfires while trying to cruise with light throttle pressure in the 2-3k rpm range. I did wiggle the ECU plugs while it was sputtering and noticed no change. The problem is still there. Any suggestions folks?

I do not understand how I used the wrong FSM. If I use the 1983 FSM, the AFM test is for the ECU with one big connector, not the ECCS type I have. Also the value range for the first test is different. 200-400. I thought that the turbo supplement for 81 was all there was. Is there an 83 turbo manual? Is it in the 83 FSM and I missed it?

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?

Go to the autoparts store and buy the tool to release the pins. I have seen them in my local NAPA store.

Ok. I missed that in the 83 FSM. I will have to check that out. The ECU plugs have been suspect to me as well. Any suggestions for replacing or repairing ECU connectors? I know the 300zx used that style also and they are rumored to be a better fit. How do you release the wire terminals from the connector?

Go to the autoparts store and buy the tool to release the pins. I have seen them in my local NAPA store.

I use a tiny flat head screwdriver to release the pins. There are small holes next to each pin to stick it into. A needle, pin or tiny paperclip will work also. If the wiggle while driving test doesnt change anything, this is not likely the culprit.

After using the 83 tests, I believe my AFM is bad. I got a reading of 120.4 and the number grew as I slowly opened the flap. Then after a the flap was open maybe a quarter inch I got an infinity reading and the numbers became .239, .283, .314, .321, etc. A couple of wiser folks agreed it was bad. Working on getting a remanufactured unit. I hope that is the end of the stumble at cruise.